A tie is a beam like structure that provides support for a track, and in the case of railroad tracks, couples or ties the rails of a train track. As FIG. 1A illustrates, which is taken from the U.S. Pat. No. 6,336,265 to Niedermair, ground based railway cross ties 100 are positioned on and supported by a rail bed 102 that is mostly comprised of crushed gravel that forms a ballast base 104 that supports the entire cross tie 100. In use, the ground based cross ties 100 are positioned in a parallel and spaced apart configuration partially submerged within the ballast 104 so that the upper surface 106 of each cross tie 100 is exposed, and the rest of their body is securely grounded and is supported by the ballast 104. Two railroad track rails 108 are secured in place to the cross ties 100 in a well known conventional manner, such as by spikes 110.
FIG. 1B illustrates a typical wooden cross tie 120 used on a railway bridge. The tie 120 rests on two steel girders 122 rather than being submerged within ballast 104. Conventionally, bridge rails 108 are placed or positioned such that the girder span (or length) LG between the girders 122 is equal to or greater than the rail span LR between the two rails 108. In other words, a horizontal distance 123 separates the outer edges 125 of the rails 108 from the inner top surface edges 127 of the girders 122. The wider girder span LG creates greater stability and therefore supports and protects (prevents) the train from flipping over from centrifugal forces during turns. The horizontal distance 123 between the rails 108 and the girders 122 (or the girder span LG) depends on engineering and construction constraints. For example, a bridge may have electric or other utility pipes that extend along the length of the bridge, and which may be laid in between the girders 122. In such an instance, the girder span LG (and hence the horizontal separation 123) would have to be sufficiently long to accommodate the placement of the utility pipes. On the other hand, a bridge might not have any utility lines or pipes, which would allow for a shorter horizontal separation distance 123 between rails 108 and girders 122.
As illustrated in FIG. 1C, under heavy load conditions, the ties 120 must withstand horizontal forces, which are generally parallel along the axis of the beam 120. The horizontal forces may include tensile forces such as those indicated by arrow A and arrow B, and/or compression forces indicated by arrow C and arrow D. The tensile and compression forces can cause horizontal shearing 124 that are generally parallel along the axis of the beam 120. As FIG. 1D illustrates, under heavy load conditions, the ties 120 must in addition withstand vertical forces that are normal to the beam 120, such as bending moments, which can cause vertical shearing 126 perpendicular to the beam 120. As illustrated, the shearing 126 is at or near where the actual load is experienced by the beam 120, which is proximal to the outer edges 125 of the rails 108 and the inner top surface edges 127 of the girders 122, where an unsupported distance 123 between the these two edges exists. Of course, the beam 120 need not bend to shear vertically. As stated in Chambers Dictionary of Science and Technology, volume 2, 1974, shearing is a “type of deformation in which parallel planes in a body remain parallel but are relatively displaced in a direction parallel to themselves; in fact, there is a tendency for adjacent planes to slide over each other. For example, a rectangle, if subjected to a shearing force parallel to one side, becomes a parallelogram. The bending moment at any imaginary transverse section of a beam is equal to the algebraic sum of the moments of all the forces to either side of the section of the beam.” Therefore, bridge cross ties 120 may differ in construction compared with ground based cross ties in that bridge cross ties must have a higher structural strength and integrity to withstand and oppose all the tensile, compression, shear, and torsion forces and bending moments that are exerted by a heavy load. With ground based cross ties, the ballast 104 bears and opposes some of these forces.
Most conventional ties (bridge or ground) have been formed from hardwood, concrete, or steel. Conventional hardwoods present disadvantages in that given their scarcity, they are expensive to produce and susceptible to decay. This is particularly true in marine environments where hardwood bridge cross ties are used on bridges that span over bodies of water. Hardwood cross ties can be treated with creosote to prolong their life span. However, creosote is toxic, which can result in potential environmental hazards.
Previous attempts have been made to develop a substitute for the conventional wooden ties, such as by manufacturing cross ties from synthetic resins, concrete, or steel. Although synthetic resins may be used as ground based cross ties, where a ballast exists as a major load bearing support, they cannot be used as bridge cross ties where no rail bed exists. Regarding concrete and steel ties, they are heavy and awkward to maneuver, difficult to install (must provide special openings for spikes), and concrete ties shatter upon impact. Both concrete and steel ties are expensive to make and repair. Furthermore, steel, standing either alone or as reinforcement in porous concrete, is subject to corrosion.
Other attempts have been made to provide long lasting ties. Reference is made to U.S. Pat. Nos. 6,336,265 and 4,150,790. Regrettably, these ties suffer from one or more disadvantages such as low bending strength, low resistance to impact loading, short life, difficult installation and/or lack of durability.